This invention relates generally to the field of cytogenetics, and more particularly to the field of molecular cytogenetics. It concerns methods of determining the relative copy numbers of different nucleic acid sequences in a subject cell or cell population and/or comparing the nucleic acid sequence copy numbers of substantially identical sequences in several cells or cell populations as a function of the location of those sequences in a reference genome. For instance, the methods of this invention provide the means to determine the relative number of copies of nucleic acid sequences in one or more subject genomes (for example, the DNA of one tumor cell or a number of cells from a subregion of a solid tumor) or portions thereof as a function of the location of those sequences in a reference genome (for example, a normal human metaphase spread). Further, the invention provides methods of determining the absolute copy number of nucleic acid sequences in a subject cell or cell population.
Although the examples herein concern human cells and the language is primarily directed to human concerns, the concept of this invention is applicable to genomes from any plant or animal. The genomes compared need only be related closely enough to have sufficient substantially identical sequences for a meaningful analysis. For example, a human genome and that of another primate could be compared according to the methods of this invention.
Chromosome abnormalities are associated with genetic disorders, degenerative diseases, and exposure to agents known to cause degenerative diseases, particularly cancer, German, xe2x80x9cStudying Human Chromosomes Today,xe2x80x9d American Scientist, 58: 182-201 (1970); Yunis; xe2x80x9cThe Chromosomal Basis of Human Neoplasia,xe2x80x9d Science, 221: 227-236 (1983); and German, xe2x80x9cClinical Implication of Chromosome Breakage,xe2x80x9d in Genetic Damage in Man Caused by Environmental Agents, Berg, Ed., pgs. 65-86 (Academic Press, New York, 1979). Chromosomal abnormalities can be of several types, including: extra or missing individual chromosomes, extra or missing portions of a chromosome (segmental duplications or deletions), breaks, rings and chromosomal rearrangements, among others. Chromosomal or genetic rearrangements include translocations (transfer of a piece from one chromosome onto another chromosome), dicentrics (chromosomes with two centromeres), inversions (reversal in polarity of a chromosomal segment), insertions, amplifications, and deletions.
Detectable chromosomal abnormalities occur with a frequency of one in every 250 human births. Abnormalities that involve deletions or additions of chromosomal material alter the gene balance of an organism and generally lead to fetal death or to serious mental and physical defects. Down syndrome can be caused by having three copies of chromosome 21 instead of the normal 2. This syndrome is an example of a condition caused by abnormal chromosome number, or aneuploidy. Down syndrome can also be caused by a segmental duplication of a subregion on chromosome 21 (such as, 21q22), which can be present on chromosome 21 or on another chromosome. Edward syndrome (18+), Patau syndrome (13+), Turner syndrome (XO) and Kleinfelter syndrome (XXY) are among the most common numerical aberrations. [Epstein, The Consequences of Chromosome Imbalance; Principles, Mechanisms and Models (Cambridge Univ. Press 1986); Jacobs, Am. J. Epidemiol, 105: 180 (1977); and Lubs et al., Science, 169: 495 (1970).]
Retinoblastoma (del 13q14), Prader-Willis syndrome (del 15q11-q13), Wilm""s tumor (del 11p13) and Cri-du-chat syndrome (del 5p) are examples of important disease linked structural aberrations. [Nora and Fraser, Medical Genetics; Principles and Practice, (Lea and Febiger (1989).]
One of the critical endeavors in human medical research is the discovery of genetic abnormalities that are central to adverse health consequences. In many cases, clues to the location of specific genes and/or critical diagnostic markers come from identification of portions of the genome that are present at abnormal copy numbers. For example, in prenatal diagnosis, as indicated above, extra or missing copies of whole chromosomes are the most frequently occurring genetic lesion. In cancer, deletion or multiplication of copies of whole chromosomes or chromosomal segments, and higher level amplifications of specific regions of the genome, are common occurrences.
Much of such cytogenetic information has come over the last several decades from studies of chromosomes with light microscopy. For the past thirty years cytogeneticists have studied chromosomes in malignant cells to determine sites of recurrent abnormality to glean hints to the location of critical genes. Even though cytogenetic resolution is limited to several megabases by the complex packing of DNA into the chromosomes, this effort has yielded crucial information. Among the strengths of such traditional cytogenetics is the ability to give an overview of an entire genome at one time, permitting recognition of structural abnormalities such as inversions and translocations, as well as deletions, multiplications, and amplifications of whole chromosomes or portions thereof. With the coming of cloning an detailed molecular analysis, recurrent translocation sites have been recognized as involved in the formation of chimeric genes such as the BCR-ABL fusion in chronic myelogeneous leukemia (CML); deletions have been recognized as frequently indicating the location of tumor suppressor genes; and amplifications have been recognized as indicating overexpressed genes.
Conventional procedures for genetic screening and biological dosimetry involve the analysis of karyotype. A karyotype is the particular chromosome complement of an individual or of a related group of individuals, as defined both by the number and morphology of the chromosomes usually in mitotic metaphase. It include such things as total chromosome number, copy number of individual chromosome types (e.g., the number of copies of chromosome X), and chromosomal morphology, e.g., as measured by length, centromeric index, connectedness, or the like. Karyotypes are conventionally determined by chemically staining an organism""s metaphase, prophase or otherwise condensed (for example, by premature chromosome condensation) chromosomes. Condensed chromosomes are used because, until recently, it has not been possible to visualize interphase chromosomes due to their dispersed condition and the lack of visible boundaries between them in the cell nucleus.
A number of cytological techniques based upon chemical stains have been developed which produce longitudinal patterns on condensed chromosomes, generally referred to as bands. The banding pattern of each chromosome within an organism usually permits unambiguous identification of each chromosome type (Latt, xe2x80x9cOptical studies of Metaphase Chromosome Organization,xe2x80x9d Annual Review of Biophysics and Bioengineering, 5: 1-37 (1976)].
Unfortunately, such conventional banding analysis requires cell culturing and preparation of high quality metaphase spreads, which is time consuming and labor intensive, and frequently difficult or impossible. For example, cells from many tumor types are difficult to culture, and it is not clear that the cultured cells are representative of the original tumor cell population. Fetal cells capable of being cultured, need to be cultured for several weeks to obtain enough metaphase cells for analysis. over the past decade, methods of in situ hybridization have been developed that permit analysis of intact cell nuclei-interphase cytogenetics. Probes for chromosome centromeres, whole chromosomes, and chromosomal segments down to the size of genes, have been developed. With the use of such probes, the presence or absence of specific abnormalities can be very efficiently determined; however, it is tedious to test for numerous possible abnormalities or to survey to discover new regions of the genome that are altered in a disease.
The present invention, Comparative Genomic Hybridization (CGH) [formerly called Copy Ratio Reverse Cytogenetics (CRRC) among other names] provides powerful methods to overcome many of the limitations of existing cytogenetic techniques. When CGH is applied, for example, in the fields of tumor cytogenetics and prenatal diagnosis, it provides methods to determine whether there are abnormal copy numbers of nucleic acid sequences anywhere in the genome of a subject tumor cell or fetal cell or the genomes from representative cells from a tumor cell population or from a number of fetal cells, without having to prepare condensed chromosome spreads from those cells. Thus, cytogenetic abnormalities involving abnormal copy numbers of nucleic acid sequences, specifically amplifications and/or deletions, can be found by the methods of this invention in the format of an immediate overview of an entire genome or portions thereof. More specifically, CGH provides methods to compare and map the frequency of nucleic acid sequences from one or more subject genomes or portions thereof in relation to a reference genome. It permits the determination of the relative number of copies of nucleic acid sequences in one or more subject genomes (for example, those of tumor cells) as a function of the location of those sequences in a reference genome (for example, that of a normal human cell).
Gene amplification is one of several mechanisms whereby, cells can change phenotypic expression when increased amounts of specific proteins are required, for example, during development [Spradling and Mahowald, PNAS (USA), 77: 1096-1100 (1980); Glover et al., PNAS (USA 79: 2947-2951 (1982)1, or during an environmental challenge when increased amounts of specific proteins@can impart resistance to cytotoxic agents [Melera et al., J. Biol. Chem. 255: 7024-7028 (1980); Beach and Palmiter, PNAS (USA, 78: 21102114 (1981)].
A major limitation of Southern analysis and related conventional techniques for analysis of gene amplification is that only specific sites are studied leaving the vast majority of the genome unekanined. Conventional cytogenetic studies, on the other hand, provide a broad survey of the genome but provide little information about genes that may be involved in amplification events. However, the procedures of this invention overcome those limitations. This invention can be used to show the normal chromosomal locations of all regions of a genome that are amplified or deleted wherein the size of the regions that can be detected is limited only by the resolution of the microscopy used and the organization of DNA in condensed chromosomes. Thus, this invention provides among other uses the ability to study gene amplifications and deletions and their roles in tumor development, progression and response to therapy more thoroughly than was possible previously. The methods of CGH are sufficiently rapid and simple that large numbers of subject nucleic acids, for example from many tumors, can be analysed in studies for gene amplification and deletion.
The karyotypic heterogeneity in solid tumors can be extreme. Identification of commonly occurring chromosomal changes by analysis of metaphase spreads is often difficult or impossible using conventional banding analysis because of the complexity of the rearrangements and because of the poor quality of the metaphase preparations. CGH overcomes that limitation in that the tumor nucleic acid can be studied without the requirement of poring metaphase spreads. Since CGH can probably be performed on single cells by amplifying the nucleic acid therefrom, CGH can be used to investigate the heterogeneity of tumors by studying representative cells from different cell populations of the tumor. Alternatively, CGH of nucleic acid from a tumor extracted in a bulk extraction process from many cells of the tumor can reveal consistencies within the apparent heterogeneity. For example, the same amplified sequences may appear as homogeneously staining regions (HSRS) and/or double minute chromosomes (DMs) in one tumor cell but as an extension of a chromosome arm in another tumor cell. Thus, order from the apparent randomness may be realized by CGH hybridization.
Montgomery et al., PNAS (USA), 80: 5724-5728 (September 1983), concerns the hybridization of labeled Cot fractionated DNAs from tumor cell lines (a Cot fraction from which the high copy repeats, low copy repeats and single copy sequences were substantially removed) to metaphase spreads from said tumor cell lines. Basically, Montgomery et al. mapped the positions of nucleic acid sequences from tumor cell lines that are very highly amplified back to tumor cell line genomes.
Total genomic DNA from one species has been used in in situ hybridization to discriminate in hybrid cells between chromosomes of that species and of a different species on the basis of the signal from the high copy repetitive sequences. [Pinkel et al., PNAS (USA), 83: 2934 (1986); Manuelidis, Hum. Genet., 71: 288 (1985); and Durnam et al., Somatic Cell Molec. Genet., 11: 571 (1985).] Landegent et al., Hum. Genet., 77. 366-370 (1987), eliminated highly repetitive sequences, like Alu and Kpn fragments, from whole cosmid cloned genomic sequences by blocking the highly repetitive sequences with Cot-1 DNA. The resulting probe was used for in situ hybridization.
European Patent Application Publication No. 430,402 published Jun. 5, 1991) describes methods and compositions for chromosome-specific painting, that is, methods and compositions for staining chromosomes based upon nucleic acid sequence employing high complexity nucleic acid probes. In general in the chromosome-specific painting methods, repetitive sequences not specific to the targeted nucleic acid sequences are removed from the hybridization mixture and/or their hybridization capacity disabled, often by blocking with unlabeled genomic DNA or with DNA enriched for high copy repetitive sequences as is Cot-1 [commercially available from Bethesda Research Laboratory, Gaithersburg, Md. (USA)]. Pinkel et al., PNAS (USA), 85: 9138-9142 (1980) also describes aspects of chromosome-specific painting as well as International Publication No. WO 90/05789 (published May 31, 1990 entitled xe2x80x9cin situ Suppression Hybridization and Uses Thereforxe2x80x9d).
Chromosome-specific repeat sequence probes and chromosome-specific painting probes can be hybridized in situ to interphase nuclei as well as metaphase spreads and provide information about the genetic state of the individual targeted genomes. A limitation of such hybridizations is that cytogenetic information is only provided from the regions to which the probes bind. Such hybridizations are very useful for determining if a particular abnormality is present, for example, the deletion of a specific gene or a duplication among other abnormalities, but it is laborious to search for currently unknown abnormalities on a region by region basis.
Other methods of searching for unknown genetic abnormalities similarly require a lot of work. For example, looking for loss of heterozygosity in tumor cells, requires the hybridization of many probes to Southern blots of tumor and normal cell DNA. The instant invention, Comparative Genomic Hybridization (CGH), provides methods to overcome many of the limitations of the existing cytogenetic techniques.
Saint-Ruf et al., Genes, Chromosomes and Cancer, 2: 18-26 (1990) state at page 24 that
Human breast carcinomas are characterized by two sets of molecular anomalies. Firstly, some protooncogenes, such as MYC, INT2, HST, and ERBB2, are frequently found either amplified or overexpressed . . . Secondly, loss of heterozygosity has been reported, especially for ip, 11, 13 and 17 . . .
Human breast carcinomas are also characterized cytogenetically by various anomalies that may be the chromosomal counterpart of the molecular anomalies: regions of amplification (HSRS) are found in more than one-third of the tumors . . . and various deletions, affecting, e.g., 1p, 11p, 11q, 13, and 17p, are found recurrently . . .
[Citations omitted.] Saint-Ruf et al. concluded from the reported experiments that although amplification of genetic material is a frequent and probably important event in breast carcinogenesis, that the relevant genes involved in such amplifications remain unknown but do not seem to correspond to the proto-oncogenes commonly considered important in breast cancer.
Since HSRs in tumors are most often not at the site of the amplified gene(s) in normal cells, standard cytogenetics does not yield any information that could assist with identification of the gene(s). CGH on the other hand permits mapping them in the normal genome, a major step towards their identification.
Dutrillaux et al., Cancer Genet. Cytogenet., 49; 203-217 (1990) report (at page 203) that xe2x80x9c[a]though human breast carcinomas are among the most frequent malignant tumors, cytogenetic data remain scarce, probably because of their great variability and of the frequent difficulty of their analysis.xe2x80x9d In their study of xe2x80x9c30 cases with relatively simple karyotypes to determine which anomalies occur the most frequently and, in particular, early during tumor progressionxe2x80x9d (p. 203), they concluded that xe2x80x9ctrisomy iq and monosomy 16q are early chromosomal changes in breast cancer, whereas other deletions and gain of 8q are clearly secondary events.xe2x80x9d [Abstract, p. 203.] Dutrillaux et al. further state (at page 216) that deletions within tumor suppressor genes xe2x80x9ccharacterize tumor progression of breast cancer.xe2x80x9d
It is believed that many solid tumors, such as breast cancer, progress from initiation to metastasis through the accumulation of several genetic aberrations. [Smith et al., Breast Cancer Res. Treat., 18 Suppl. 1: S 514 (1991); van de Vijver and Nusse, Biochim. Biophys. Acta, 1072: 33-50 (1991); Sato et al., Cancer Res., 50; 7184-7189 (1990).] Such genetic aberrations, as they accumulate, may confer proliferative advantages, genetic instability and the attendant ability to evolve drug resistance rapidly, and enhanced angiogenesis, proteolysis and metastasis. The genetic aberrations may affect either recessive xe2x80x9ctumor suppressor genesxe2x80x9d or dominantly acting oncogenes. Deletions and recombination leading to loss of heterozygosity (LOH) are believed to play a major role in tumor progression by uncovering mutated tumor suppressor alleles.
Dominantly acting genes associated with human solid tumors typically exert their effect by overexpression or altered expression. Gene amplification is a common mechanism leading to upregulation of gene expression. [Stark et al., Cell, 75: 901-908 (1989).] Evidence from cytogenetic studies indicates that significant amplification occurs in over 50% of human breast cancers. [Saint-Ruf et al., supra.] A variety of oncogenes have been found to be amplified in human malignancies. Examples of the amplification of cellular oncogenes in human tumors is shown in Table 1 below.
SOURCE: modified from Varmus, Ann. Rev. Genetics, 18: 553-612 (1984) [cited in Watson et al., MoleculaL Biology of the Gene (4th ed.; Benjamin/Cummings Publishing Co. 1987)]
Chromosomal deletions involving tumor suppressor genes may play an important role in the development and progression of solid tumors. The retinoblastoma tumor suppressor gene (Rb-1), located in chromosome 13q14, is the most extensively characterized tumor suppressor gene (Friend et al., Nature, 323: 643 (1986); Lee et al., Science, 235: 1394 (1987); Fung et al., Science, 236: 1657 (1987)]. The Rb-1 gene product, a 105 kDa nuclear phosphoprotein, apparently plays an important role in cell cycle regulation [Lee et al., supra (1987); Howe et al., PNAS USA), 87: 5883 (1990)]. Altered or lost expression of the Rb protein is caused by inactivation of both gene alleles either through a point mutation or a chromosomal deletion. Rb-i gene alterations have been found to be present not only in retinoblastomas [Friend et al., supra (1986); Lee et al., supra (1987); Fung et al., supra (1987)] but also in other malignancies such as osteosarcomas [Friend et al., supra (1986)], small cell lung cancer [Hensel et al., Cancer Res., 50: 3067 (1990); Rygaard et al., Cancer Res., 50: 5312 (1990)] and breast cancer [Lee et al., Science, 241: 218 (1988); T""Ang et al., Science, 242: 263 (1988); Varley et al., Oncogene, 4: 725 (1989)]. Restriction fragment length polymorphism (RFLP) studies have indicated that such tumor types have frequently lost heterozygosity at 13q suggesting that one of the Rb-1 gene alleles has been lost due to a gross chromosomal deletion [Bowcock et al., Am. J. Hum. Genet., 46: 12 (1990)].
The deletion of the short arm of chromosome 3 has been associated with several cancers, for example, small cell lung cancer, renal and ovarian cancers; it has been postulated that one or more putative tumor suppressor genes is or are located in the p region of chromosome 3 (ch. 3p) [Minna et al., Symnosia on Quantitative Biology, Vol. LI: 843-853 (SCH Lab 1986); Cohen et al., N. Eng. J. Med., 301: 592-595 (1979); Bergerham et al., Cancer Res., 49: 13901396 (1989); Whang-Peng et al., Can. Genet. Cytogenet.,II: 91-106 (1984; and Trent et al., Can. Genet. Cytogenet., 14: 153-161 (1985)].
The above-indicated collection of amplified and deleted genes is far from complete. As the Saint-Ruf et al. study (supra) of oncogene amplification in cells showing cytogenetic evidence of amplification, such as double minutes (DMs) or homogeneously staining regions (HSRs), indicated, the amplified genes were not known oncogenes in most cases. As Dutrillaux et al., supra indicated, xe2x80x9ccytogenetic data remains scarcexe2x80x9d for xe2x80x9cthe most frequent malignant tumorsxe2x80x9dxe2x80x94breast carcinomas.
Discovery of genetic changes involved in the development of solid tumors has proven difficult. Karyotyping is impeded by the low yield of high quality metaphases and the complex nature of chromosomal changes [Teyssier, J. R., Cancer Genet. Cytogenet., 31: 103 (1989)]. Although molecular genetic studies of isolated tumor DNA have been more successful and permitted detection of common regions of allelic loss, mutation or amplification [Fearon et al., Cell, 61: 759 (1990); Sato et al., Cancer Res., 50: 7184 (1990); Alitalo et al., Adv. Cancer Res., 47: 235 (1986); and Schwab and Amler, Genes Chrom. Cancer., 1: 181 (1990)], such molecular methods are highly focused, targeting one specific gene or chromosome region at a time, and leaving the majority of the genome unexamined.
Thus, a research tool leading to the identification of amplified and deleted genes and providing more cytogenetic data regarding tumors, especially tumor progression and invasiveness is needed in tumor cytogenetics. CGH provides such a molecular cytogenetic resrch tool.
CGH facilitates the genetic analysis of tumors in that it provides a copy number karyotype of the entire genome in a single step. Regions of tumor DNA gain and loss are mapped dirdctiy onto normal chromosomes. comparisons of primary tumors with their metastses by CGH should be informative concerning cancer progression.
The ability to survey the whole genome in a single hybridization is a distinct advantage over allelic loss studies by restriction fragment length polymorphism (RFLP) that target only one locus at a time. RFLP is also restricted by the availability and informativeness of polymorphic probes.
The copy number karyotype determined by CGH may become as important for diagnostic and/or prognostic assessment of solid tumors as conventional karyotyping now is for hematologic malignancies. [Yunis, J. J., Science, 221: 227 (1983); Solomon et al., Science, 254: 1153 (1991).]
Comparative Genomic Hybridization (CGH) employs the kinetics of in situ hybridization to compare the copy numbers of different DNA or RNA sequences from a sample, or the copy numbers of different DNA or RNA sequences in one sample to the copy numbers of the substantially identical sequences in another sample. In many useful applications of CGH, the DNA or RNA is isolated from a subject cell or cell population. The comparisons can be qualitative or quantitative. Procedures are described that permit determination of the absolute copy numbers of DNA sequences throughout the genome of a cell or cell population if the absolute copy number is known or determined for one or several sequences. The different sequences are discriminated from each other by the different locations of their binding sites when hybridized to a reference genome, usually metaphase chromosomes but in certain cases interphase nuclei. The copy number information originates from comparisons of the intensities of the hybridization signals among the different locations on the reference genome.
Two representative basic approaches are employed in CGH as illustrated herein for the analysis of subject DNAS. In an example of the first approach, genomic DNA from a subject cell or cell population of cells is isolated, labeled and hybridized to reference chromosomes, usually in metaphase. In an example of the second approach, genomic DNAs from two or more subject cells or cell populations are isolated, differentially labeled, and hybridized to reference chromosomes, usually in metaphase!
The CGH methods of this invention can be qualitative and/or quantitative. A particular utility of CGH is for analysing DNA sequences from subject cell(s) or cell populations, for example from clinical specimens including tumor and fetal tissues.
An important utility of CGH is to find regions in normal genomes which when altered in sequence copy number contribute to disease, as for example, cancer or birth defects. For example, regions at elevated copy number may contain oncogenes, and regions present at decreased copy number may contain tumor suppressor genes.
A representative CGH method is for comparing copy numbers of different DNA sequences in a subject cell or cell population comprising the steps of:
a) extracting the DNA from the subject cell or from a number of cells of the subject cell population;
b) amplifying said extracted subject DNA, if necessary;
c) labeling the subject DNA;
d) hybridizing said labeled subject DNA in situ to reference metaphase chromosomes after substantially removing from the labeled DNA those repetitive sequences that could bind to multiple loci in the reference metaphase chromosomes, and/or after blocking the binding sites for those repetitive sequences in the reference metaphase chromosomes by prehybridization with appropriate blocking nucleic acids, and/or blocking those repetitive sequences in the labeled DNA by prehybridization with appropriate blocking nucleic acid sequences, and/or including such blocking nucleic acid sequences for said repetitive sequences during said hybridization, wherein the DNA sequences in the labeled subject DNA that bind to single copy sequences in the reference metaphase chromosomes are substantially retained, and those single copy DNA sequences as well as their binding sites in the reference metaphase chromosomes remain substantially unblocked both before and during the hybridization;
e) rendering the bound, labeled DNA sequences visualizable, if necessary;
f) observing and/or measuring the intensity of the signal from the labeled subject DNA sequences as a function of position on the reference metaphase chromosomes; and
g) comparing the copy numbers of different DNA sequences of the subject DNA by comparing the signal intensities at different positions on the reference metaphase chromosomes, wherein the greater the signal intensity at a given position, the greater the copy number of the sequences in the subject DNA that bind at that position. An analogous method can be performed wherein the subject nucleic acid is RNA.
Further, disclosed are methods wherein two or more subject nucleic acids are analysed by CGH. Exemplary methods are those wherein the subject nucleic acids are DNA sequences from a subject cell or cell population. Analogous methods may be performed wherein the subject nucleic acids are RNA. Such an exemplary method is that for comparing copy numbers of different DNA sequences in one subject cell or cell population relative to copy numbers of substantially identical sequences in another cell or cell population, said method comprising the steps of:
a) extracting the DNA from both of the subject cells or cell populations;
b) amplifying said extracted subject DNAS, if necessary;
c) differentally labeling the subject DNAS;
d) hybridizing said differentially labeled subject.
DNAs in situ to reference metaphase chromosomes after substantially removing from the labeled DNAs those repetitive sequences that could bind to multiple loci in the reference metaphase chromosomes, and/or after blocking the binding sites for those repetitive sequences in the reference metaphase chromosomes by prehybridization with appropriate blocking nucleic acids, and/or blocking those repetitive sequences in the labeled DNA by prehybridization with appropriate blocking nucleic acid sequences, and/or including such blocking nucleic acid sequences for said repetitive sequences during said hybridization;
e) rendering the bound, differentially labeled DNA sequences visualizable, if necessary;
f) observing and/or measuring the intensities of the signals from each subject DNA, and the relative intensities, as a function of position along the reference metaphase chromosomes; and
g) comparing the relative intensities among different locations along the reference metaphase chromosomes wherein the greater the intensity of the signal at a location due to one subject DNA relative to the intensity of the signal due to the other subject DNA at that location, the greater the copy number of the sequence that binds at that location in the first subject cell or cell population relative to the copy number of the substantially identical sequence in the second subject cell or cell population that binds at that location.
Further disclosed are methods of quantitatively comparing copy numbers of different DNA sequences in one subject cell or cell population relative to copy numbers of substantially identical sequences in another subject cell or cell population. A representative method is that comprising steps (a) through (e) of the method immediately detailed above and the following steps of:
f. measuring the intensities of the signals from each of the bound subject DNAs and calculating the ratio of the intensities as a function of position along the reference metaphase chromosomes to form a ratio profile; and
g. quantitatively comparing the ratio profile among different locations along the reference metaphase chromosomes, said ratio profile at each location being proportional to the ratio of the copy number of the DNA sequence that bind to that location in the first subject cell or cell population to the copy number of substantially identical sequences in the second cell or cell population.
Said representative methods can further comprise comparing copy numbers of different DNA sequences in more than two subject DNAs wherein the comparing is done pairwise between the signals from each subject DNA.
This invention further discloses methods to determine the ratio of copy numbers of different DNA sequences in one subject cell or cell population to copy numbers of substantially identical sequences in another cell or cell population wherein the steps of (a) through (f) as described above are performed as well as the following steps:
g. determining the average copy number of a calibration sequence in both subject cells or cell populations, said calibration sequence being substantially identical to a single copy sequence in the reference metaphase cells; and
h. normalizing the ratio profile calculated in (f) so that at the calibration position, the ratio profile is equal to the ratio of the average copy numbers determined in (g), the normalized ratio profile at any other location along the reference metaphase chromosomes thereby giving the ratio of the copy numbers of the DNA sequences in the two subject DNAs that bind at that location. That method can be extended to further subject nucleic acids as for example determining the ratio of copy numbers of DNA sequences in more than two subject DNAs wherein the comparing is done pairwise between signals from each subject DNA.
Further disclosed are methods for comparing copy numbers of different DNA sequences in a test cell or cell population, said method comprising applying steps (a) through (e) of the above-described methods and
f. observing and/or measuring the intensities of the signal from each subject DNA, and the relative intensities, as a function of position along the reference metaphase chromosomes wherein one of the subject cells or cell populations is the test cell or cell population and the other is a normal cell or cell population; and
(g) comparing the relative intensities among different locations along the reference metaphase chromosomes, wherein the greater the relative intensity at a location, the greater the copy number of the sequence in the test cell or cell population that binds to that location, except for sex chromosomes where the comparison needs to take into account the differences in copy numbers of sequences in the sex chromosomes in relation to those on the autosomes in the normal subject cell or cell population.
A related representative method is that for comparing the copy number of different DNA sequences in a test cell or cell population comprising applying steps (a) through (e) of the above described methods wherein one of the subject cells or cell populations is the test cell or cell population, and the other is a standard cell or cell population wherein the copy numbers of the DNA sequences that bind to different positions on the reference metaphase chromosomes is known and steps:
f. measuring the intensities of the signals from each of the bound subject DNAs and calculating the ratio of intensities as a function of position along the reference metaphase chromosomes to form a ratio profile;
g. adjusting the ratio profile at each location along the reference metaphase chromosomes by multiplying the ratio profile by the known copy number of DNA sequences in the standard cell or cell population that bind there; and
h. comparing the adjusted ratio profiles at different locations along the reference metaphase chromosomes wherein the greater the adjusted ratio profile at a location, the greater the copy number of the DNA sequence in the test cell or cell population that binds there.
Another related representative method is that for determining the ratios of the copy numbers of different DNA sequences in a test cell or cell population, said method comprising applying steps (a) through (f) of the immediately above-described method and the steps of adjusting the ratio profile at each location along the reference metaphase chromosomes by multiplying the ratio profile by the known copy number of sequences that bind there; and calculating the ratio of the copy number of a DNA sequence in the test cell or cell population that binds to one location on the reference metaphase chromosomes to the copy number of a sequence that binds to another location by dividing the adjusted ratio profile at the location of the first sequence by that at the location of the second. Said representative method can be extended to determine the copy number of different DNA sequences in a test cell or cell population wherein steps (a) through (f) as described above are followed and then the following steps of adjusting the ratio profile at each location along the reference metaphase chromosomes by multiplying the ratio profile by the known copy number of DNA sequences in the standard cell or cell population that bind there;
determining the copy number of a calibration sequence in the test cell or cell population that is substantially identical to a single copy sequence in the reference cells; and
normalizing the adjusted ratio profile so that at the location of the calibration sequence on the reference metaphase chromosomes, the normalized, adjusted ratio profile is equal to the copy number of the calibration sequence determined in the above step, the value of the normalized, adjusted ratio profile at another location then being equal to the copy number of the DNA sequence in the test cell or cell population that binds at that location. That method can be analogously performed wherein two or more calibration sequences are used, and the adjusted ratio profile is normalized to get the best fit to the copy numbers of the ensemble of calibration sequences. Preferably, the copy number of the calibration sequence is determined by in situ hybridization. Those methods can comprise in situ hybridizing probes for more than one calibration position and normalizing to obtain the best fit of the ratio profile to the calibration positions. The standard cell or cell population preferably have normal genomes. In many applications of CGH, the reference metaphase chromosomes are normal.
Further, this invention concerns the use of antenna cell lines. An exemplary method is for detecting amplification of a certain sequence or group of sequences in a subject cell or cell population, comprising essentially steps (a) through (e) of the above-described methods wherein the in situ hybridization is targeted to antenna cells in which the DNA sequences to be tested for is or are amplified, and examining the reference cell for regions that are hybridized significantly more intensely than others, the presence of such regions indicating amplifications of the sequences which are being tested. The chromosomes of said antenna cell lines may be in interphase or in metaphase.
When a single labeled subject nucleic acid is being hybridized, or if multiple labeled subject nucleic acids are hybridized sequentially, it is important that the binding sites on the reference genome not be saturated prior to observing and/or measuring the signal intensity(ies). In the case of a single labeled subject nucleic acid, nonsaturation can be effected in a number of ways, for example, by stopping the hybridization, by providing insufficient subject nucleic acid, and/or by providing a sufficient amount of unlabeled nucleic acid which is sufficiently complementary to the reference chromosomes to competitively prevent saturation of sites therein by the labeled subject nucleic acid.
When there are two or more labeled subject nucleic acids, those subject nucleic acids can be hybridized in situ to the reference genome sequentially or simultaneously. simultaneous in situ hybridization is preferred in that saturation of the targeted binding sites in the reference genome will not interfere with the procedure. When sequential in situ hybridization is used, it must be performed under conditions wherein the individual hybridizations are stopped well before the binding sites on the reference chromosomes are saturated.
Objects of this invention are to detect sequence copy number imbalances throughout an entire genome in one hybridization, to map gains and/or losses of sequences in a genome, and/or to provide a copy number karyotype of a subject genome.
Further, an object of this invention is to enable the detection of relative copy number differences that are common to a number of different cells and/or cell populations. For example, CGH methods can be used wherein DNAs extracted from cells of many different tumors are combined and labeled; the hybridization of those combined labeled DNAs to normal condensed chromosomes, provides for the rapid identification of only those copy number changes that occurred in most of the tumors. Less frequently occurring variations would be averaged out. Thus, this invention further provides for a CGH method wherein two or more of the subject nucleic acids that were extracted from different cells and/or from numbers of cells from different cell populations, are labeled the same, and hybridized to a reference spread under conditions wherein repetitive sequences are removed and/or suppressed and wherein sequence copy number differences that are common in said combined labeled nucleic acid sequences are determined.
Another object of this invention is to provide the means of cytogenetically analysing archived chromosomal material, that is, fixed material from, for example, biopsied tissue specimens, preferably cataloged and keyed to medical records of patients from whom the specimens were taken, and archaeological chromosomal material. Such chromosomal material cannot, of course, be karyotyped according to traditional means in that no live cells are present to culture and from which to prepare chromosomal spreads. However, the nucleic acid can be extracted therefrom and amplified by a polymerase chain reaction (PCR) procedure or by a non-PCR procedure and tested by the methods of this invention.
This invention further provides for a method to detect simultaneously an ensemble of amplifications and/or deletions in a tumor wherein the results can be used to determine the subsequent behavior of that tumor. Said determination is made by associating the patterns of amplifications and/or deletions in tumor cells with the behavior of that tumor. Such associations can be made by testing, for example, as indicated immediately above, DNA from archived tumor tissue keyed to medical records, or when fresh tumor specimens are tested by CGH and the patients are followed. Further, such associations can be made with CGH methods wherein there are more than one subject cell and/or cell population, for example, one or more tumors.
Another object of this invention is to provide a method of analyzing cells from a suspected lesion at an early stage of development. An advantage of the methods of this invention is that only a few cells are necessary for the analysis. The early detection of amplifications and/or deletions in cells from a lesion allow for early therapeutic intervention that can be tailored to the extent of, for example, invasiveness known to be associated with such genetic rearrangements. Further, such early detection provides a means to associate the progression of the cells with the genetic rearrangements therein detected by the methods of this invention.
Tumors can be karyotypically heterogeneous containing therein various populations of cells each having different types of genetic rearrangements. As indicated above tumor cells are difficult to culture, and it is not clear that cultured cells are representative of the original tumor cell population. This invention provides the means to by-pass the culturing obstacle and allows genetic characterization of tumor cells and thus, of the heterogeneity of tumors by testing cells from different subregions thereof according to the methods of this invention. Bulk extraction of the nucleic acid from many cells of a tumor can also be used to test for consistent amplifications and/or deletions within a tumor.
It is another object of this invention to provide methods of detecting amplifications and/or deletions of nucleic acid sequences wherein certain cell lines termed herein xe2x80x9cantenna cell linesxe2x80x9d, are used to enhance the sensitivity of the detection.
It is still further an object of this invention to provide methods of prenatal or perinatal analysis wherein the nucleic acid of the child""s cells is extracted and tested according to the methods of this invention. In one embodiment of CGH, such material is human and hybridized to a normal human metaphase spread to detect whether any deletions and/or amplifications are therein present, for example, an extra copy of chromosome 21, diagnostic for Down syndrome. Test kits for performing CGH methods are also provided.